Semiconductor laser driving apparatus and optical disc apparatus including the same

ABSTRACT

A semiconductor laser driving apparatus for directing light to an optical disc for recording a recording mark based on a recording current and reproducing the recording mark to generate a reproduction signal. The apparatus includes a reproduction current generation section; a high frequency current generation section for generating a current including a high frequency component for reducing semiconductor laser noise in the reproduction; a recording current generation section, the recording current including a pulse corresponding to the recording mark and including a plurality of multi-pulses; and a current driving section for amplification. The apparatus further includes a filter to attenuate the enhanced high frequency components in the high frequency current and in the enhanced high frequency component; and a switching section for switching the filter so that the enhanced component in the recording current is superposed on at least one of the multi-pulses.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor laser drivingapparatus used for recording and reproducing data in an optical discapparatus, and an optical disc apparatus including the same.

[0003] 2. Description of the Related Art

[0004] Recently, an optical disc is expected to be used as a recordingmedium of video data replacing video tapes, in addition to an externalrecording medium for a personal computer. Video data encompasses anenormous amount of data. A high recording density is needed to recordsuch an enormous amount of data on a small-diameter optical disc. Thefollowing is available as means for achieving a high recording density.

[0005] Means 1: introduction of a PWM (pulse width modulation) system,by which information its recorded as the positions of edges of recordingmarks.

[0006] Means 2: introduction of recording correction technology forcorrecting positions of edges of recording marks so as to reduce a markedge shift which occurs due to thermal interference between therecording marks at the time of recording.

[0007] Means 3: reduction in the level of noise generated in areproduction system.

[0008] For a conventional rewritable optical disc medium, a PPM (pulseposition (or phase) modulation) system is generally used, according towhich information is recorded as positions of recording marks. Accordingto the PWM system, unlike the PPM system, information is recorded aspositions of edges of recording marks. Therefore, one recording mark catprovide two pieces of information. This is advantageous for increasingthe recording density. However, it is necessary to precisely control thepositions of the edges, which in turn requires precise control ofpositions of pulses of a recording current for recording the recordingmarks and also requires precise control of balance of the amount of heatwhich acts on the recording marks.

[0009]FIG. 13 shows recording pulses 133 conventionally used for the PWMsystem and recording marks 132 which are recorded by the pulses 133 onthe optical disc. A recording current 131 includes the pulses 133 forrecording the recording marks 132 and a bias power current 136. Eachpulse 133 includes a plurality of multi-pulses 134. Each multi-pulse 134Includes a peak power level 135 and a trough power level 137.

[0010] As shown in FIG. 13, each pulse 133 has peak power levels 135(high level) and trough power levels 137 (low level) appearingalternately. In the case where recording is performed on an optical discIncluding a phase shift recording layer, a phase shift phenomenon isutilized to perform recording. More specifically, each mark 132 (i.e.,amorphous state) is recorded by the peak power levels 135 of thecorresponding pulse 133, and a space (i.e., crystalline state) isrecorded by the bias power current 136 between the pulses 133. Thetrough power levels 137 of each multi-pulse 134 is used to raise thecooling speed of the recording layer.

[0011] When the density of the recording marks 13 is increased by thePWM system, each recording mark 132 is shorter than a spot diameter (notshown) of the semiconductor laser. Thermal interference between therecording marks 132 occurs. As a result, the positions of edges of therecording marks 132 deviate from the positions at which the edges shouldbe placed. In order to avoid this, it has been proposed to correct thepulses 133 of the recording current 131 in consideration of thedeviation in the positions of edges of the recording marks 132 asdescribed above as means 2.

[0012] When the recording marks 132 are shorter than the spot diameterof the semiconductor laser, an amplitude of a reproduction signalgenerated from the recording marks 132 is restricted by an opticalresolution of light emitted from the semiconductor laser and is thusreduced. FIG. 14 is a graph illustrating the relationship between theamplitude C (carrier level) of a reproduction signal and the frequency Fof recording marks of different lengths, i.e., a longer recording markand a shorter recording mark. The shorter recording mark is recorded ata higher density than the longer recording mark. The relationship isobtained by measurement performed on a spectrum analyzer. As shown inFIG. 14. as the length of the recording mark is decreased, the amplitudeof the reproduction signal generated from the recording mark isdecreased, in proportion to the optical resolution of light emitted fromthe semiconductor laser. In consideration that it is necessary to obtaina sufficient signal-to-noise ratio oven at a high recording density,reduction of the level of noise included in the reproduction signal isrequired in correspondence with the reduction in the amplitude of thereproduction signal. The noise included in the reproduction signal froman optical disc includes a semiconductor laser driving current noise NKwhich is generated in a semiconductor laser driving circuit for drivingthe semiconductor laser, a semiconductor laser noise NL generated in thesemiconductor laser, and a media no,se ND caused by the shape of agroove on the optical disc.

[0013] In conventional optical disc apparatuses, the semiconductor lasernoise NL occupies a very large part of the noise included in thereproduction signal. In order to improve the quality of high densitydata recording, it is demanded to provide a semiconductor laser drivingapparatus for reducing the semiconductor laser noise NL.

[0014] Hereinafter, an exemplary conventional optical semiconductorlaser driving apparatus including a semiconductor laser drivingapparatus will be described.

[0015]FIG. 15 is a schematic diagram illustrating a structure of aconventional optical disc apparatus 1500.

[0016] The optical disc apparatus 1300 includes a spindle motor 3 forrotating an optical disc 1, an optical pickup 2 for executing recordingof a recording mark on the optical disc 1 or reproduction of therecording mark recorded on the optical disc 1, and a control block 7 forcontrolling the optical pickup 2 and the motor 3. The control block 7includes a recording signal processing block 4, a reproduction signalprocessing block 5, and a central processing block 6. The centralprocessing block 6 includes a laser timing control section 61 and aformatting section 62.

[0017] The optical disc apparatus 1500 operates in the following manner.

[0018] The optical disc 1 is rotated in a certain direction by thespindle motor 3. The optical pickup 2 receives gate signals A, B, C andD. The gate signals A, B, C and D cause a semiconductor laser (not shownin FIG. 15) in the optical pickup 2 to emit light based on pulses of arecording current. In other words, recording marks are recorded based onthe pulses of the recording current. The gate signals A, B, C and D willbe described in detail later. The optical pickup 2 also opticallydetects the recording marks recorded on the optical disc 1, converts therecording marks into electric signals F, G, H, and I, and outputs theelectric signals F, G, H and I to the reproduction signal processingblock 5.

[0019] The central processing block 6 controls the reproduction signalprocessing block 5 using a control signal J, and controls the recordingsignal processing block 4 using a control signal K. The centralprocessing block 6 also controls a rotation on speed of the spindlemotor 3 using a rotation speed control signal L. The optical discapparatus 1500 further includes an interface (not shown) for connectionwith an external device.

[0020]FIG. 16 is a schematic diagram illustrating a structure of theoptical pickup 2. FIG. 16 mainly shows a portion of the optical pickup 2for converting an. electric signal into an optical signal and convertingan optical signal into an electric signal, and portions related thereto.

[0021] The optical pickup 2 includes a semiconductor laser 21 fordirecting light toward the optical disc 1 (FIG. 15) so as to recordrecording marks on the optical disc 1 based on the recording current orreproduce recording marks recorded on the optical disc 1 based on areproduction signal. The optical pickup 2 further includes asemiconductor laser driving apparatus 22 for driving the semiconductorlaser 21 and a photodetector unit 521 for reproducing the recordingmarks based on the light from the semiconductor laser 21 which isreflected by the optical disc 1. The photodetector unit 521 includesphotodetectors 23 for detecting the reflected light and converting thedetected light into a detection signal N, and a head amplifier 24 forgenerating a reproduction signal based on the detection signal N.

[0022] The optical pickup 2 operates as follows.

[0023] The semiconductor laser 21 receives a driving current M from thesemiconductor laser driving apparatus 22 and converts the drivingcurrent M into light.

[0024] For recording the recording marks, the semiconductor laser 21 isprovided with e recording current which has been modulated into pulsesas the driving current M by the semiconductor laser driving apparatus22.

[0025] For generating a reproduction signal for reproducing therecording marks recorded on the optical disc 1, the semiconductor laser21 is supplied with a reproduction current (DC current) as the drivingcurrent M by the semiconductor laser driving apparatus 22. The current Kis converted into light and output from the semiconductor laser 21. Thelight from the semiconductor laser 21 is reflected by the optical disc1. Depending on whether there is a recording mark or not, the amount,the polarization angle or the phase of the reflected light changes. Thischange is detected by the photodetector 23 and converted into thedetection current N. The detection current N is converted into a voltageby the head amplifier 24 by I-V conversion. and thus becomes areproduction signal including focus signals F and G and tracking signalsH and I. The reproduction signal is supplied to the reproduction signalprocessing block 5. Then, the reproduction signal processing block 5executes focus/tracking control. By adding the focus signals F and G andthe tracking signals H and I and extracting a high frequency component,pit information Corresponding to the recording marks 132 recorded on theoptical disc 1 is reproduced.

[0026]FIG. 17 is a schematic diagram illustrating a structure of thesemiconductor laser driving apparatus 22.

[0027] The semiconductor laser driving apparatus 22 includes a recordingand reproduction current generation section 518, a high-frequencycurrent generation section 519, and a current driving section 511.

[0028] The recording and reproduction current generation section 518includes current switching blocks 501 through 504, a reproduction powercurrent source 505, a peak power current source 506, a bias powercurrent source 507, and a trough power current source 508, and anaddition block 510.

[0029] The high frequency current generation section 519 includes a highfrequency superposition control section 512, an AC power supply 513 anda capacitor 514. The high frequency current generation section 519generates a high frequency current including a high frequency componentfor reducing the level of semiconductor laser noise included in thereproduction signal.

[0030] The current switching block 501 switches a reproduction currenton or off. The current switching block 502 switches on or off a peakcurrent included in the recording current. The current switching block503 switches on or off a bias current included in the recording current.The currant switching block 504 switches on or off a trough currentincluded in the recording current.

[0031] The reproduction current, the peak current, the bias current andthe trough current which are switched on or off by the current switchingblocks 501 through 504 are respectively provided by the reproductionpower current source 505, the peak power current source 506, the biaspower current source 507 and the trough power current source 508. Valuesof the reproduction current, the peak current, the bias current and thetrough current are respectively set by a reproduction power settingsignal O, a peak power setting signal P, a bias power setting signal Qand a trough power setting signal R in accordance with predeterminedlaser powers (reproduction power, peak power, bias power, and troughpower).

[0032] The gate signals A through D mentioned above are, morespecifically, a reproduction power gate signal A input to the currentswitching block 501, a peak power gate signal B input to the currentswitching block 502, a bias power gate signal C input to the currentswitching block 503, and a trough power gate signal D input to thecurrent switching block 504. Whether or not the value settings of thereproduction current, the peak current, the bias current and the troughcurrent is made effective is respectively determined by the gate signalsA through D.

[0033] The reproduction current, the peak current, the bias current andthe trough current generated in this manner are synthesized by theaddition block 510 into a recording current having pulses. The recordingcurrent is amplified by the current driving section 511 into the drivingcurrent M.

[0034] The high frequency superposition control block 512 is connectedto the addition bloc 510 through the AC power supply 513 and thecapacitor 514. For reproduction, the high frequency superpositioncontrol block 512 superposes a high frequency current including a highfrequency component which is substantially 300 MHz on a reproductionsignal for driving the semiconductor laser 21. The high frequencycomponent included in the high frequency current is preferably 300 MHz,which is about 10 times the reproduction frequency band.

[0035] The high frequency component, which is substantially 300 MHz,superposed on the reproduction current reduces the semiconductor lasernoise NL generated by “mode hopping” and improves the signal-to-noiseratio of the reproduction signal. Whether the superposition is preformedor not (i.e., superposition/non-superposition switching) is controlledby a high frequency superposition gate signal E which is applied to thehigh frequency superposition control block 512.

[0036] The current driving section 511 has a frequency characteristicwhich enhances a high frequency component included in a high frequencycurrent generated by the high frequency current generation section 519at the time of reproduction and enhances a high frequency componentincluded in a recording current generated by the recording andreproduction current generation section 518 at the time of recording.

[0037] Hereinafter, an operation of the conventional semiconductor laserdriving apparatus 22 will be described with reference to FIG. 18.

[0038]FIG. 18 it a timing diagram of an optical output from thesemiconductor laser 21 (FIG. 16), the reproduction power gate signal A,the peak power gate signal B, the bias power gate signal C. the troughpower gate signal D, and the high frequency superposition gate signal E.FIG. 18 also shows a waveform of the optical output (i.e.,reproduction/recording current M) from the semiconductor laser 21. Inthe example shown in FIG. 18. the gate signals A through E are set to beactive at a high (H) level.

[0039] When the semiconductor laser driving apparatus 22 begins areproduction operation, the reproduction power gate signal A is placedin an active state (i.e., a high level), and the semiconductor laser 21starts emitting light based on a reproduction current 1873 (DC signal).Since the high frequency superposition gate signal E is simultaneouslyplaced in an active state, a high frequency current 1875 of 100 MHz ormore, for example, in the vicinity of 300 MHz, is superposed on thereproduction current 1873 as the driving signal M of the semiconductorlaser 21. Therefore, the semiconductor laser 21 emits light based on arecording pulse current shown in FIG. 18 which is obtained as a resultof superposing the high frequency current 1875 on the reproductioncurrent 1873.

[0040] For recording, the level of each of the peak power gate signal B,the bias power gate signal C, and the trough power gate signal D changesin accordance with the pattern of the recording marks to be recorded.Based on the peak power gate signal B, the bias power gate signal C, andthe trough power gate signal D, a recording current 1674 iscorresponding to the pattern of the recording marks to be recorded isgenerated by the above-described operation of the reproduction powercurrent source 505 (FIG. 17), the peak power current source 506, thebias power current source 507, the trough power current source 508, andthe addition block 510. The current driving section 511 enhances thehigh frequency component included in the generated recording current1874.

[0041] The semiconductor laser 21 emits light having pulses which aresubstantially the same as those of the recording current 1874, thusrecording the recording marks on the optical disc 1 (FIG. 15).

[0042] Power values of the reproduction power current source 505, thepeak power current source 506, the bias power current source 507 and thetrough power current source 508 have the relationship of thereproduction power current source 505<the trough power current source508<the bias power current source 507<the peak power current source 506.The power values are set in the following manner.

[0043] Where currents used for forming the driving current M to besupplied to the semiconductor laser 21 by the reproduction power settingsignal O, the peak power setting signal P, the bias power setting signalQ and the trough power setting signal R are respectively currents IO,IP, IQ and IR, the power value of the reproduction power current Source505 is set so as to correspond to the value of the current IO. The powervalue of the trough power current source 508 is set so as to correspondto the value of the currents (IO+IR). The power value of the bias powercurrent source 507 is set so as to correspond to the value of thecurrents (IO+IR+IQ). The power value of the peak power current source506 is set so as to correspond to the value of the currents(IO+IR+IQ+IP). The power values are set by superposing the currents inthis manner.

[0044] An optical disc apparatus for recording data on a phase shiftrecording layer by the PWM system using the above-obtained recordingcurrent is described in Nikkei Electronics, No. 7.00 (publication date:Oct. 6, 1997).

[0045] The optical disc apparatus 1500 including the semiconductor laserdriving apparatus 22 shown in FIG. 17 suffers from the followingproblems when actually operating.

[0046] Problem 1: The shape of the recording marks recorded on theoptical disc is not uniform due to dispersion in the composition of therecording layer of the optical disc.

[0047] Problem 2: The shape of the recording marks recorded on theoptical disc is not uniform due to fluctuations in linear velocity oftracks of the optical disc with respect to the optical pickup occurringduring recording.

[0048] Problem 3: is the case of high density recording, optimumrecording is not realized merely by correcting the positions of theedges of each recording mark. It is also necessary to correct the powervalue of the recording power for each recording mark.

[0049] Problem 4: A high frequency signal superposed on the reproductionsignal generates unnecessary radiation of high frequency noise to theoutside of the semiconductor laser is driving apparatus.

[0050] Problem 5: Provision of a low pass filter for the purpose ofreducing the semiconductor laser driving current noise NK results in thewaveform of the recording current being less sharp.

[0051] The above-mentioned problems will be described below.

[0052] Problem 1

[0053] In the case of an optical disc utilizing a phase shiftphenomenon, recording marks are recorded as follows. The temperature ofa first portion of the recording layer in which a recording mark is tobe recorded is rapidly raised with laser power of more than a certainlevel, thereby placing the first portion in an amorphous state. Thetemperature of a second portion of the recording layer in which arecording mark is not to be recorded is gradually raised with laserpower of a lower level than that used in the first portion, therebyplacing the second portion in a crystalline state. The level oftemperature rise which is necessary to realize the amorphous statevaries in accordance with parameters including the thermal absorptionratio and thermal diffusion constant.

[0054]FIG. 19 shows a shape of a recording mark 132A recorded on arecording layer having a high thermal absorption ratio using the pulses133 (FIG. 13) of the recording current 131. As shown in FIG. 19, aleading edge 132B of the recording mark 132A is not normally shaped andtapered.

[0055] In the case of the PPM system, the information is recorded as theposition of each recording mark as described above. Therefore, taperingof the recording mark does not greatly influence the reproductionsignal. In the case of the PWM system, by contrast, tapering of therecording mark generates the following inconvenience. The shape of theleading edge 132B is not normal with the positions of the edges beingdeviated from the intended positions, and thus Jitter is increased atthe leading edge 1323. As a result, an error rate of the reproductionsignal is raised.

[0056] Problem 2

[0057] When the linear velocity of tracks on the optical disc withrespect to the optical pickup is increased due to the fluctuations inthe rotation speed of the optical disc, the level of the temperaturerise of the recording layer caused by the pulses 133 (FIG. 13) isreduced. As a result, the recording mark 132A shown in FIG. 19 havingthe tapered leading edge 132B is obtained. Thus, an error rate of thereproduction signal is increased.

[0058] Problem 3

[0059] According to the PWM system, the balance of the amount of heatwhich acts on the recording marks is precisely controlled by usingmulti-pulses as described above.

[0060] When a 3T mark (T−width of detection window) shown in FIG. 20 isrecorded, the following problem occurs. The 3T mark has a pulse width of3T, which is the shortest possible pulse width realized by 8-16modulation. The pulse width 3T (i.e., the length of the recording mark)is shorter than a diameter DI of a spot of light directed by thesemiconductor laser. In an experiment performed by the presentinventors, the 3T marks actually recorded were longer than intended. Thepresent inventors found that in order to record a 3T mark properly, itis necessary for a pulse width W2 of a multi-pules 134A for recordingthe 3T mark to be shorter than a pulse width W1 of a multi-pulse 134 forrecording a 4T or longer mark (e.g., a 5T mark shown in FIG. 20).

[0061] When the pulse width W2 is shorter, the temperature of therecorded 3T mark itself does not reach a thermal saturation point, As aresult, the recorded 3T mark is unstable is terms of shape.

[0062] Problem 4

[0063] It has been reported that a high frequency component (included ina high frequency current) of about 300 MHz to about 450 MHz during areproduction operation is effective in reducing the semiconductor lasernoise NL included in a reproduction signal. A high frequency currentincluding such a high frequency component preferably has the largestpossible amplitude. The reason is that as the amplitude of such a highfrequency current is larger, the duty of the waveform of light emittedby the semiconductor laser is reduced, namely, the time period in whichthe semiconductor laser does not emit light is extended. Therefore,interference of the light emitted by the semiconductor laser to anoptical disc and the light returning to the semiconductor laser afterbeing reflected by the optical disc can be alleviated. Thus, modehopping in the semiconductor laser is more unlikely to occur, and thesemiconductor laser noise NL included in the reproduction signal isreduced. According to an experiment performed by the present inventors,the amplitude of a high frequency current effective for reducing thesemiconductor laser noise NL was 50 mApp at 300 MHz.

[0064] However, when a high frequency current of 300 MHz having anamplitude of 50 mApp is transmitted, a loss in the amplitude isgenerated during the transmission, as a result of which the highfrequency current has a smaller amplitude when being input to thesemiconductor laser. In order to provide the semiconductor laser with ahigh frequency current having a larger amplitude, the current value ofthe high frequency current generated by the high frequency currentgeneration section 519 (FIG. 17) can be increased. However, thisincreases unnecessary radiation of high frequency noise to the outsideof the semiconductor laser driving apparatus, which is a problem withrespect to safety standards.

[0065] In order to enhance the high frequency component included in thehigh frequency current at the time of reproduction, the current drivingsection 511 may be designed to have a frequency characteristic having afrequency peak at 300 MHz. FIG. 21 shows a waveform of a recordingcurrent generated when the current driving section 511 has such afrequency characteristic (waveform (b)) in comparison to a waveform of anormal recording current (waveform (a)). The waveform (a) is obtainedwhen the current driving section 511 does not have a frequencycharacteristic having such a frequency peak. The recording current shownby waveform (b) has excessive overshoot and undershoot respectively atthe rising and falling of the pulses. Therefore, it is impossible toprovide a recording mark having proper recording characteristics.However, due to restrictions on circuit design or the like, it is verydifficult to provide the current driving section 511 with a frequencycharacteristic which is flat from a low frequency to a high frequency of300 MHz.

[0066] Problem 5

[0067] Problem 5 occurs when a driving current for driving thesemiconductor laser itself includes a noise component (semiconductorlaser driving current noise NK), not the semiconductor laser noise NLcaused by the semiconductor laser. Since the amplitude of a reproductionsignal is reduced as the recording density is increased as describedabove. it is demanded to significantly reduce the level of noise.

[0068] When a low pass filter is provided for cutting out noise in areproduction signal band, the level of noise in the reproduction signalcan be reduced in a reproduction operation. FIG. 22 shows a waveform ofa recording current which has passed through a low pass filter (waveform(b)) in comparison to a waveform of a normal recording current (waveform(a)). Pulses of the recording current shown by waveform (b) has bluntededges as compared to the edges of the recording current shown bywaveform (a). Therefore, the recording sensitivity is lowered.

SUMMARY OF THE INVENTION

[0069] According to one aspect of the invention, a semiconductor laserdriving apparatus, for driving a semiconductor laser for directing lightto an optical disc for recording a recording mark on the optical discbased on a recording current and reproducing the recording mark recordedon the optical disc so as to generate a reproduction signal, includes areproduction current generation section for generating the reproductioncurrent; a high frequency current generation section for generating ahigh frequency current including a high frequency component for reducingsemiconductor laser noise included in the reproduction signal; arecording current generation section for generating the recordingcurrent, the recording current including a pulse corresponding to therecording mark and the pulse including a plurality of multi-pulses; anda current driving section for amplifying the reproduction current andthe recording current. The high frequency component included in the highfrequency current generated by the high frequency current generationsection is enhanced at the time of reproduction on, and the highfrequency component included in the recording current generated by therecording current generation section is enhanced at the time ofrecording. The semiconductor laser driving apparatus further includes afilter for operating so as to attenuate the enhanced high frequencycomponent included in the high frequency current generated by the highfrequency current generation section and the enhanced high frequencycomponent included in the recording current generated by the recordingcurrent generation section; and a switching section for switching thefilter on or off so that the enhanced high frequency component includedin the recording current is superposed on at least one of the pluralityof multi-pulses included in the pulse of the recording current.

[0070] In one embodiment of the invention, the current driving sectionhas a frequency characteristic for enhancing the high frequencycomponent, and the current driving section enhances the high frequencycomponent included in the high frequency current generated by the highfrequency current generation section at the time of reproduction andenhances the high frequency component included in the recording currentgenerated by the recording current generation section at the time ofrecording.

[0071] In one embodiment of the invention, the switching sectionincludes a switch connected to the filter and a timing control sectionfor controlling the timing of opening or closing of the switch.

[0072] In one embodiment of the invention, the at least one of theplurality of multi-pulse includes a leading multi-pulse.

[0073] In one embodiment of the invention, the pulse includes a specificpulse having a specific pattern, and the switching section causes thefilter to operate so that the enhanced high frequency component includedin the recording current is superposed on the specific pulse.

[0074] In one embodiment of the invention, the recording mark includes a3T mark recorded by 8-16 modulation. The specific pulse includes a 3Tpulse corresponding to the 3T mark. The switching section causes thefilter to operate so that the enhanced high frequency component includedin the recording current is superposed on the 3T pulse.

[0075] In one embodiment of the invention, the switching section causesthe filter to operate so that the enhanced high frequency componentincluded in the recording current is superposed on a portion of at leastone of the plurality of multi-pulses included in the pulse.

[0076] In one embodiment of the invention, the switching section causesthe filter to operate so that the enhanced high frequency componentincluded in the recording current is superposed on an entirety of atleast one of the plurality of multi-pulses included in the pulse.

[0077] In one embodiment of the invention, the at least one of theplurality of multi-pulses includes a trailing multi-pulse.

[0078] In one embodiment of the invention, the switching section causesthe filter to operate so that the enhanced high frequency componentincluded in the recording current is superposed on all of the pluralityof multi-pulses included in the pulse.

[0079] In one embodiment of the invention, the reproduction current is aDC current.

[0080] In one embodiment of the invention, the switching section causesthe filter to operate so that the enhanced high frequency componentincluded in the high frequency current is superposed on the reproductioncurrent at the time of reproduction, and causes the filter to operate sothat the enhanced high frequency component included in the recordingcurrent is attenuated at the time of recording.

[0081] In one embodiment of the invention, the high frequency componenthas a frequency of 100 MHz or higher.

[0082] In one embodiment of the invention, the high frequency componenthas a frequency of 100 MHz or higher and 450 MHz or lower.

[0083] In one embodiment of the invention, the high frequency componenthas a frequency of substantially 300 MHz.

[0084] In one embodiment of the invention, the filter includes a highpass filter.

[0085] In one embodiment of the invention, the high frequency componenthas a frequency which is higher than a out-off frequency of the filter.

[0086] In one embodiment of the invention, the filter includes aplurality of filter circuits having different frequency characteristicsand different impedance values from one another.

[0087] In one embodiment of the invention, the switching section selectsone of the plurality of filter circuits as a filter which operates basedon a linear velocity of the optical disc.

[0088] According to another aspect of the invention, an optical discapparatus includes an optical pickup for recording a recording mark onan optical disc and reproducing the recording mark recorded on theoptical disc; a motor for rotating the optical disc; and a control blockfor controlling the optical pickup and the motor. The optical pickupincludes a semiconductor laser for directing light to the optical discfor recording the recording mark on the optical disc based on arecording current and reproducing the recording mark recorded on theoptical disc so as to generate a reproduction current, and asemiconductor laser driving apparatus for driving the semiconductorlaser. The semiconductor laser driving apparatus includes a reproductioncurrent generation section for generating the reproduction current, ahigh frequency current generation section for generating a highfrequency current including a high frequency component for reducing asemiconductor laser noise included in the reproduction current, arecording current generation section for generating the recordingcurrent, the recording current including a pulse corresponding to therecording mark and the pulse including a plurality of multi-pulses, anda current driving section for amplifying the reproduction current andthe recording current, The high frequency component included in the highfrequency current generated by the high frequency current generationsection is enhanced at the time of reproduction, and the high frequencycomponent included in the recording current generated by the recordingcurrent generation section is enhanced at the time of recording. Thesemiconductor laser driving apparatus further includes a filter foroperating so as to attenuate the high frequency component included inthe high frequency current generated by the enhanced high frequencycurrent generation section and the enhanced high frequency componentincluded in the recording current generated by the current signalgeneration section. and a switching section for switching the filter onor off so that the enhanced high frequency component included in therecording current is superposed on at least one of the plurality ofmulti-pulses included in the pulse of the recording current.

[0089] In one embodiment of the invention, the current driving sectionhas a frequency characteristic for enhancing the high frequencycomponent, and the current driving section enhances the high frequencycomponent included in is the high frequency current generated by thehigh frequency current generation section at the time of reproductionand enhances the high frequency component included in the recordingcurrent generated by the recording current generation section at thetime of recording.

[0090] In one embodiment of the invention, the control block includes alinear velocity detection section for detecting a linear velocity of theoptical disc. The switching section causes the filter to operate so thatthe enhanced high frequency component included in the recording currentis superposed on at least one of the plurality of multi-pulses includedin the pulse based on the linear velocity of the optical disc.

[0091] In one embodiment of the invention, the linear velocity detectionsection detects the linear velocity of the optical disc based on thereproduction current.

[0092] In one embodiment of the invention, the linear velocity detectionsection detects the linear velocity of the optical disc based on arotation speed of the motor.

[0093] In one embodiment of the invention, the linear velocity detectionsection detects the linear velocity of the optical disc based on aradial position of the optical pickup on the optical disc.

[0094] Thus, the invention described herein makes possible theadvantages of providing a semiconductor laser driving apparatus forproviding stable recording characteristics at the time of high densityrecording and a reproduction current having a high signal-to-ratio, andan optical disc apparatus including such a semiconductor laser drivingapparatus as a result of solving the above-described problems.

[0095] These and other advantages of the present invention will becomeapparent to those skilled in the art upon reading and understanding thefollowing detailed description with reference to the accompanyingfigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0096]FIG. 1 is a schematic diagram illustrating a structure of anoptical disc apparatus according to an example of the present invention;

[0097]FIG. 2 is a schematic diagram illustrating a structure of anoptical pickup included in the optical disc apparatus shown in FIG. 1;

[0098]FIG. 3 is a schematic diagram illustrating a structure of asemiconductor laser driving apparatus included in the optical pickupshown in FIG. 2:

[0099]FIG. 4 is a schematic diagram illustrating a structure of a filterin the semiconductor laser driving apparatus shown in FIG. 3;

[0100]FIG. 5 is a graph illustrating a frequency characteristic of thefilter shown in FIG. 4;

[0101]FIG. 6 is a timing diagram of control signals used for driving thesemiconductor laser driving apparatus according to the presentinvention, which also shows resulting recording pulses and recordingmarks;

[0102]FIG. 7 is a schematic diagram illustrating another structure of afilter and components related thereto of a semiconductor laser drivingapparatus according to the present invention;

[0103]FIG. 8 is another timing diagram of control signals used fordriving a semiconductor laser driving apparatus according to the presentinvention, Which also shows resulting recording pulses and recordingmarks;

[0104]FIG. 9 is still another timing diagram of a control signal usedfor driving a semiconductor laser driving apparatus according to thepresent invention, which also shows resulting recording-pulses andrecording marks;

[0105]FIG. 10 is a schematic diagram illustrating still anotherstructure of a filter and components related thereto of a semiconductorlaser driving apparatus according to the present invention;

[0106]FIG. 11A is a graph illustrating a frequency characteristic of thefilter shown in FIG. 10;

[0107]FIG. 11B is a graph illustrating a frequency characteristic ofanother filter usable in a semiconductor laser driving apparatusaccording to the present invention;

[0108]FIG. 12 is still another timing diagram of a control signal usedfor driving a semiconductor laser driving apparatus according to thepresent invention, which also shows resulting recording pulses andrecording marks;

[0109]FIG. 13 shows recording marks and a waveform of recording pulsesin a conventional semiconductor laser driving apparatus;

[0110]FIG. 14 is a graph illustrating a frequency characteristic of acarrier signal and noise components when recording on an optical disc isperformed;

[0111]FIG. 15 is a schematic diagram illustrating a structure of aconventional optical disc apparatus;

[0112]FIG. 16 is a schematic diagram illustrating a structure of anoptical pickup included in the conventional optical disc apparatus shownin FIG. 15;

[0113]FIG. 17 is a schematic diagram illustrating a structure of asemiconductor laser driving apparatus included in the optical pickupshown in FIG. 16;

[0114]FIG. 18 is a timing chart of control signals used for driving aconventional semiconductor laser driving apparatus;

[0115]FIG. 19 shows recording marks and a waveform of recording pulsesin a conventional semiconductor laser driving apparatus;

[0116]FIG. 20 shows recording marks and a waveform of recording pulsesin a conventional semiconductor laser driving apparatus at the time ofhigh density recording;

[0117]FIG. 21 show normal recording pulses of a conventionalsemiconductor laser driving apparatus in comparison to recording pulsesof a conventional semiconductor laser driving apparatus having afrequency peak; and

[0118]FIG. 22 shows normal recording pulses of a conventionalsemiconductor laser driving apparatus in comparison to recording pulsesof a conventional semiconductor laser driving apparatus obtained by alow pass filter.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0119] Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

[0120]FIG. 1 is a schematic diagram illustrating a structure of anoptical disc apparatus 100 according to an example of the presentinvention. Identical elements previously discussed with respect to FIG.15 bear identical reference numerals and the detailed descriptionsthereof will be omitted.

[0121] The optical disc apparatus 100 includes a spindle motor 3 forrotating an optical disc 1, an optical pickup 2A for recording arecording mark on the optical disc 1 or reproducing the recording markrecorded on the optical disc 1, and a control block 7A for controllingthe optical pickup 2A and the motor 3. The control block 7A includes arecording signal processing block 4A, a reproduction signal processingblock 5A, and a central processing block 6A. The central processingblock 6A includes a linear velocity detection section 63, a laser timingcontrol section 61, a switching timing control section 65, a formattingsection 62, and a linear velocity setting section 64. The recordingsignal processing block 4A includes a data pattern detection section 41.The reproduction signal processing block 5A includes a recordingcharacteristic detection section 51.

[0122]FIG. 2 is a schematic diagram illustrating a structure of theoptical pickup 2A. Identical elements previously discussed with respectto FIG. 16 bear identical reference numerals and the detaileddescriptions thereof will be omitted.

[0123] The optical pickup 2A includes a semiconductor laser 21 fordirecting light toward the optical disc 1 so as to record the recordingmarks on the optical disc 1 based on the recording current or reproducethe recording marks recorded on the optical disc 1 based on areproduction current. The optical pickup 2A further includes asemiconductor laser driving apparatus 122 for driving the semiconductorlaser 21 and a photodetector unit 521 for reproducing the recordingmarks based on the light from the semiconductor laser 21 reflected bythe optical disc 1. The photodetector unit 521 includes at least onephotodetector 23 for detecting the reflected light and converting thedetected light into a detection signal N, and a head amplifier 24 forgenerating a reproduction signal based on the detection signal N.

[0124]FIG. 3 is a schematic diagram illustrating a structure of thesemiconductor laser driving apparatus 122. Identical elements previouslydiscussed with respect to FIG. 17 bear identical reference numerals andthe detailed descriptions thereof will be omitted.

[0125] The semiconductor laser driving apparatus 122 includes arecording and reproduction current generation section 518, a highfrequency current generation section 519, a current driving section 511,a filter 515, and a switching section 520.

[0126] The recording and reproduction current generation section 510includes current switching blocks 501 through 504, a reproduction powercurrent source 505, a peak power current source 506, a bias powercurrent source 507, and a trough power current source 508, and anaddition block 510.

[0127] The high frequency current generation section 519 includes a highfrequency superposition control section 512, an AC power supply 513 anda capacitor 514. The high frequency current generation section 519generates a high frequency current including a high frequency componentfor reducing the level of a semiconductor laser noise included in thereproduction current. The switching section 520 includes a switch 516and a timing control section 517,

[0128] The current switching block 501 switches a reproduction currenton or off. The current switching block 502 switches on or off a peakcurrent included in the recording current. The current switching block503 switches on or off a bias current included in the recording current.The current switching block 504 switches on or off a trough currentincluded in the recording current.

[0129] The reproduction current, the peak current, the bias current andthe trough current which are switched on or off by the current switchingblocks 501 through 504 are respectively provided by the reproductionpower current source 505, the peak power current source 506, the biaspower current source 507 and the trough power current source 508. Valuesof the reproduction current, the peak current, the bias current and thetrough current are respectively set by a reproduction power settingsignal O, a peak power setting signal P, a bias power setting signal Qand a trough power setting signal R in accordance with predeterminedlaser powers (reproduction power, peak power, bias power, and troughpower).

[0130] The gate signals A through D are a reproduction power gate signalA input to the current switching block 501, a peak power gate signal Binput to the current switching block 502, a bias power gate signal Cinput to the current switching block 503, and a trough power gate signalD input to the current switching block 504. Whether or not the valuesettings of the reproduction current, the peak current, the bias currentand the trough current is made effective is respectively determined bythe gate signals A through D.

[0131] The reproduction current, the peak current, the bias current andthe trough current generated in this manner are synthesized by theaddition block 510 into a recording current having pulses. The recordingcurrent is amplified by the current driving section 511 into the drivingcurrent M.

[0132] The high frequency superposition control block 512 is connectedto the addition block 510 through the AC power supply 513 and thecapacitor 514. For reproduction, the high frequency superpositioncontrol block 512 superposes a high frequency current including a highfrequency component which is substantially 300 MHz on a reproductionsignal for driving the semiconductor laser 21.

[0133] The high frequency component, which is substantially 300 MHz,superposed on the reproduction current reduces the semiconductor lasernoise NL generated by mods hopping and improves the signal-to-noiseratio of the reproduction signal. Whether the superposition is performedor not (i.e., superposition/non-superposition switching) is controlledby a high frequency superposition gate signal E which is applied to thehigh frequency superposition control block 512.

[0134] The current driving section 511 has a frequency characteristicwhich enhances a high frequency component included in a high frequencycurrent generated by the high frequency current generation section 519at the time of reproduction, and enhances a high frequency componentincluded in a recording current generated by the recording andreproduction current generation section 518 at the time of recording.

[0135] Unlike the conventional semiconductor laser driving apparatus 22,the semiconductor laser driving apparatus 122 according to the presentinvention includes the filter 515, and the switching section 520including the switch 516 and the timing generation section 517 forsupplying the switch 516 with a switch timing signal S for switching theswitch 516 on or off.

[0136] The filter 515 operates so as to attenuate a high frequencycomponent included in the high frequency current enhanced by the currentdriving section 511 and a high frequency component included in therecording current enhanced by the current driving section 511. Theswitching section 520 causes the filter 515 to operate so that the highfrequency component included in the recording current enhanced by thecurrent driving section 511 is superposed on at least one of a pluralityof multi-pulses included in the recording current.

[0137] The switch 516 is placed is an open state when the switch timingsignal S is at a high level (H) and is placed in a through(shortcircuited) state when the switch timing signal S is at a low level(L). A state where the filter 515 has a high frequency componentsuperposition action on the driving current M (a recording current in arecording operation and a reproduction current is a reproductionoperation) and a state where the filter 515 does not have such an actionis switched by the switch 516.

[0138] As shown in FIG. 4, the filter 515 includes a high pass filterincluding a resistor and a capacitor. When the switch 516 is in ashortcircuited state (i .e., the switch timing signal S is at a lowlevel), a current having a frequency component equal to or higher than acut-off frequency FC (i.e., 1/(2IIRC) ) passes through the filter 515and thus bypasses the semiconductor laser 21. when the switch 516 is inan open state (i.e., the switch timing signal S is at a high level). acurrent having any frequency component in any band including thefrequency component equal to or higher than the cut-off frequency FCpasses through the semiconductor laser 21. The cut-off frequency FC isdetermined by a resistance R of the resistor and a capacitance C of thecapacitor of the filter 515.

[0139]FIG. 5 is a graph illustrating an example of a frequencycharacteristic of the filter 515 (relationship between the impedance Zand the frequency F). At a given frequency level, as the impedance ofthe filter 515 is decreased, the amount of the current which passesthrough the filter 515 is increased and thus the amount of the currentwhich passes through the semiconductor laser 21 is decreased.

[0140] For reproduction, the semiconductor laser driving apparatus 22supplies the semiconductor laser 21 with a reproduction current (DCcurrent). For recording, the semiconductor laser driving apparatus 22supplies the semiconductor laser 21 with a recording current havingpulses in correspondence with the pattern of the recording marks (i.e.,data) to be recorded.

[0141] Hereinafter. an operation of the semiconductor laser drivingapparatus 122 will be described with reference to FIG. 6.

[0142]FIG. 6 is a timing diagram of an optical output from thesemiconductor laser 21 (FIG. 3), and the reproduction power gate signalA, the peak power gate signal B, the bias power gate signal C, thetrough power gate signal A, the high frequency superposition gate signalE, and a switch timing signal S. FIG. 6 also shows a waveform of theoptical output (i.e., reproduction/recording current) from thesemiconductor laser 21. The waveform of the optical output issubstantially the same as that of the driving current M1 including thehigh frequency component superposed by the filter 515 (FIG. 3). In thisexample, it is assumed that, for example, a signal band necessary forreproducing the recording marks recorded on the optical disc 1 (FIG. 1)is 20 MHz or less, a maximum modulation frequency of the recordingcurrent is 60 MHz, and the high frequency superposition frequency is 300MHz.

[0143] The operation of the semiconductor laser driving apparatus 122 isfundamentally the same as that of the conventional semiconductor laserdriving apparatus 22 described with reference to FIG. 18 up until thecurrent driving section 511 outputs a driving current M. In thisexample, the gate signals A through E are set to be active at a high (H)level.

[0144] When the semiconductor laser driving apparatus 122 begins areproduction operation, the reproduction power gate signal A is placedin an active state, and the semiconductor laser 21 starts emitting lightbased on a reproduction current 73 (DC signal). Since the high frequencysuperposition gate signal E is simultaneously placed in an active state,a high frequency current 75 of 100 MHz or more, for example, in thevicinity of 300 MHz, is superposed on the reproduction current 73 as thedriving current M1 of the semiconductor laser 21. Therefore, thesemiconductor laser 21 emits light based on a recording pulse currentshown is FIG. 6 which is obtained as a result of superposing the highfrequency current 75 on the reproduction current 73.

[0145] For recording, the level of each of the peak power gate signal B,the bias power gate signal C, and the trough power gate signal D changesin accordance with the pattern of the recording marks to be recorded.Based on the peak power gate signal S. the bias power gate signal C, andthe trough power gate signal D, a recording current 74 corresponding tothe pattern of the recording marks to be recorded is generated by theabove-described operation of the reproduction power current source 505(FIG. 3), the peak power current source 506, the bias power currentsource 507, the trough power current source 508, and the addition block510. The current driving section 511 enhances the high frequencycomponent included in the generated recording current 74.

[0146] The filter 515 operates to attenuate the high frequency componentincluded in the recording current 74 enhanced by the current drivingsection 511. The switching section 520 causes the filter 515 to operateso that the high frequency component included in the recording current74 enhanced by the current driving section 511 is superposed on at leastone of a plurality of multi-pulses included in the recording current 74,as described is detail hereinafter.

[0147] With reference to FIG. 6, the recording current 74 has pulses 72respectively corresponding to recording marks 71. Each pulse 72 includesa plurality of multi-pulses P1 and P2. “P1” lndicates a leadingmulti-pulse among the plurality of multi-pulses. “P2” indicatesmulti-pulses other than the leading multi-pulse among the plurality ofmulti-pulses.

[0148] As shown in FIG. 6, a leading pulse B1 of the peak power gatesignal B (corresponding to the leading multi-pulse P1 of the recordingcurrent 74) is detected. Simultaneously with generation of the leadingpulse B1, the signal S is switched to a high level. Thus, the switch 516is placed in an open state, and therefore currents having frequencycomponents in all of the bands including the frequency component higherthan the cut-off frequency FC pars through the semiconductor laser 21.As a result, a high frequency component 76 enhanced by the currentdriving section 511 is superposed on the multi-pulse P1 corresponding tothe leading pulse B1 of the peak power gate signal 1 (referred to as an“overshoot”). In this specification, a high frequency component refersto a frequency component which is higher than the cut-off frequency ofthe filter 515. Also in this specification, the frequency level of thehigh frequency component is higher than the frequency level of pulses B1and B2 of the peak power gate signal B.

[0149] When the pulses B2 (corresponding to the non-leading pulses P2 ofthe recording current 74) are generated, the switch timing signal S isat a low level. Thus, the switch 516 is in a shortcircuited state, andtherefore a current having a high frequency component passes through thefilter 515 and bypasses the semiconductor laser 21. Therefore, theabove-mentioned overshoot is not generated for the multi-pulses P2corresponding to the pulses B2. As shown in FIG. 5, the level ofimpedance corresponding to frequency FA (300 MHz) (indicated by “ZA”) issufficiently small to avoid overshoot.

[0150] As described above, according to the present invention, theswitching section 520 causes the filter 515 to operate so that the highfrequency component 76 included in the recording current enhanced by thecurrent driving section 511 is superposed on the leading multi-pulse P1of the plurality of multi-pulses, based on the switch timing signal S.The switching section 520 also causes the filter 515 so that the highfrequency component 76 included in the recording current enhanced by thecurrent driving section 511 is attenuated in correspondence with thenon-leading multi-pulses P2, based on the switch timing signal S.Accordingly, overshoot is not generated for the non-leading multi-pulsesP2.

[0151] A driving current M1 obtained as a result of frequencycharacteristic adjustment is supplied to the semiconductor laser 21(FIG. 3). The semiconductor laser 21 emits light having pulses which aresubstantially the same as those of the recording current 74. thusrecording the recording marks 71 on the optical disc 1,

[0152] In the above description, the switch timing signal S iscontrolled so that the high frequency component 76 is superposed on theentire leading multi-pulse P1. The present invention is not limited tothis. The switch timing signal S can be controlled so that the highfrequency component 76 is superposed on a portion of the leadingmulti-pulse P1. Instead of generating overshoot or undershoot for theleading multi-pulse P1, overshoot or undershoot can be generated for atrailing multi-pulse or a heat-shielding pulse following the trailingmulti-pulse based on the composition of the recording layer of theoptical disc and the shape of the light spot, so that the recordingcharacteristics of the recording marks can be improved. There can becases where it is effective to generate overshoot or undershoot for allof the multi-pulses, and further there can be cases where the recordingcharacteristics of the recording marks can be improved by blunting theshape of the pulses by attenuating the high frequency component includedin the recording current enhanced by the current driving section 511.

[0153] In addition, the operation of the switch timing signal S can beadjusted based on the recording characteristics of the optical disc, sothat overshoot or undershoot can be generated for a specific multi-pulseof the recording current which has a specific pattern. Specifically, theoperation of the switch timing signal S is adjusted so as to superpose ahigh frequency component on the specific multi-pulse or attenuate a highfrequency component in correspondence with the specific multi-pulse. Therecording characteristics of the optical disc can be detected based on areproduction signal using the recording characteristic detection section51 (FIG. 1).

[0154] Hereinafter, the function and specific embodiments of the presentinvention for solving the above-described problems of the conventionalart will be described.

[0155] With reference to FIG. 6, according to the present invention, theswitching section 520 causes the filter 515 to operate so that the highfrequency component 76 included in the recording current enhanced by thecurrent driving section 511 is superposed on the leading multi-pulse P1of the plurality of multi-pulses so as to generate overshoot. Therefore,the apparent temperature rise at a leading edge of the recording mark 71is larger than that in the case where the high frequency component isnot superposed on the leading multi-pulse and thus overshoot is notgenerated. Therefore, the inconvenience that the recording mark istapered at the leading edge due to the insufficient temperature risedescribed above in the section entitled <Problem 1> is solved. As shownin FIG. 6, the recording mark 71 has a normal shape.

[0156] The phenomenon described above in the section entitled <Problem2> that the recording mark is tapered at the leading edge when thelinear velocity of the tracks of the optical disc fluctuates withrespect to the optical pickup is also solved by the same function.Namely, the switching section 520 causes the filter 515 to operate sothat the high frequency component 76 included in the recording currentenhanced by the current driving section 511 is superposed on the leadingmulti-pulse P1 of the plurality of multi-pulses so as to generateovershoot. Therefore, the apparent temperature rise at a leading edge ofthe recording mark 71 is larger. Thus, the inconvenience that therecording mark is tapered at the leading edge due to the insufficienttemperature rise is solved. The cut-off frequency FC and the impedance Zof the filter 515 can be set in accordance with the linear velocity ofthe optical disc with respect to the optical pickup.

[0157] The moving linear velocity of the recording layer, i.e., thelinear velocity of the optical disc 1 (FIG. 1) with respect to theoptical pickup 2A, is set by controlling the spindle motor 3 using arotation speed control signal L from the linear velocity setting section64 included in the central processing block 6A. The linear velocity ofthe optical disc 1 can be detected by the linear velocity detectionsection 63 included is the central processing block 6A based on areproduction signal J output from the reproduction signal processingblock 5A.

[0158] The present invention is not limited to this. The linear velocityof the optical disc 1 can alternatively be detected based on a rotationspeed of the spindle motor 3 which is detected by a rotation speeddetection section 77 (see FIG. 1). Still alternatively, the linearvelocity of the optical disc 1 can be detected based on a radialposition of the semiconductor laser 21 (included in the optical pickup2A).on the optical disc 1 which is detected by a radial positiondetection section 7B (see FIG. 1). When the detected linear velocity isequal to or more than, or equal to or less than, a predetermined level,the switch timing signal S is controlled as described above withreference to FIG. 6. In this manner, the high frequency component 76included in the recording current 74 enhanced by the current drivingsection 511 is superposed on the leading multi-pulse P1 among theplurality of multi-pulses, as a result of which overshoot is generated.Thus, the apparent temperature rise at the leading edge of the recordingmark 71 is larger than that in the case where the high frequencycomponent is not superposed on the leading multi-pulse and thusovershoot is not generated. This way, the phenomenon of the recordingmark being tapered at the leading edge is alleviated.

[0159]FIG. 7 is a block diagram illustrating an alternative structure ofa filter and components related thereto. As shown in FIG. 7, a pluralityof filters (three filter circuits 515A, 515B and 515C are shown in FIG.7) can be provided instead of the filter 515 (FIG. 3). A switchingsection 520 includes switches 516A, 516B and 516C respectivelycorresponding to the filter switches 515A, 515B and 515C, and a timingcontrol section 517A.

[0160] In the case where the frequency characteristics and the impedancevalues of the three filter circuits 515A, 515D and 515C are differentfrom one another, the following effects are provided.

[0161] In the case of an optical disc of a CAV system, the linearvelocity of the optical disc with respect to the optical pickup variesin accordance with the radial position on the optical disc of the lightdirected to the optical disc by the semiconductor laser 21. Depending onwhether the light from the semiconductor laser 21 is at a position in aninner portion, a central portion or an outer portion of the opticaldisc, one of the filter circuits 515A, 515B or 515C which has an optimumfilter constant is operated. The one of the filter circuits 515A, 5153or 515C is selected by respectively controlling the switches 516A, 516Band 516C so as to be on or off by the timing control section 517A. inthis manner, a recording current having pulses optimum for the linearvelocity of the inner, central or outer portion of the optical disc ofthe CAV system can be supplied to the semiconductor laser 21.

[0162] <Problem 3> described above is also solved by the presentinvention. The cause of problem 3 is that the pulse width of therecording current for recording a 3T mark is too short to be suppliedwith a sufficient amount of heat. According to the present invention, asshown in FIG. 8, overshoot is generated at a pulse 72A for recording a3T mark 71A, so that the amount of heat supplied to the 3T mark 71A isincreased. Thus, the 3T mark 71A or a mark shorter than the other marks(e.g. 5T mark) can be stably recorded.

[0163] The amount of heat is adjusted is the following manner. First, adata pattern of the 3T mark 71A is detected by the data patterndetection section 41 (FIG. 1; 3T mark detection section) included in therecording signal processing block 4A. More specifically, the datapattern detection section 41 detects a data pattern for recording the 3Tmark 71A from a series of modulated data patterns by means of patternmatching.

[0164]FIG. 8 shows a waveform of a data pattern detection signal U (3Tpattern detection signal) for controlling the data pattern detectionsection 41 (FIG. 1), a waveform of a reproduction and recording current73A for causing the semiconductor laser 21 to emit light, and a waveformof a switch timing signal S.

[0165] The data pattern detection section 41 detects the data patternfor recording the 3T mark 71A as described above. The switch timingsignal S is switched to a high level at a timing fulfilling an ANDcondition of the 3T pattern detection signal and the peak power gatesignal B (FIG. 3) (i.e., the condition by which both the 3T patterndetection signal and the peak power gate signal B are at a high level).Then, the switch 516 is placed in an open state. Overshoot occurs on thepulse 72A for recording the 31 mark 71A, thus increasing the totalamount of heat to be supplied to the 3T mark 71A. Owing to theovershoot, even a recording mark which is shorter than the otherrecording marks can be stably recorded.

[0166] The above-mentioned effect is also provided for an entirety of aportion of a pulse, included in a recording current, having a specificpattern for recording marks including a 4T mark and a 5T mark, bycausing overshoot to the entirety or the portion of such a pulse. Apulse having a specific pattern is also detected by the data patterndetection section 41.

[0167] <Problem 4> (i.e., the unnecessary radiation of high frequencynoise to the outside of the semiconductor laser driving apparatus causedby the high frequency superposition) is solved by the switching thelevel of the switch timing signal S between a reproduction operation anda recording operation as shown in FIG. 9.

[0168] During the recording operation, the switch timing signal S is ata low level, and thus the switch 516 (FIG. 3.) is switched on.Accordingly, the filter 515 operates so as to attenuate the highfrequency component included in the recording current enhanced by thecurrent driving section 511. An a result, as shown in FIG. 9, a pulse72B included in a recording current 74B is prevented from overshooting.

[0169] During the reproduction operation, the switch timing signal S isat a high level, and thus the switch 516 (FIG. 3) is switched off.Accordingly, the filter 515 does not operate so as to attenuate the highfrequency component included in the recording current enhanced by thecurrent driving section 511. As a result, the high frequency componentincluded a high frequency current 75B enhanced by the current drivingsection 511 is supplied to the semiconductor laser 21 without beingattenuated by the filter 515.

[0170] As described above, the high frequency current 75B can besupplied to the semiconductor laser 21 without deteriorating theamplitude of the high frequency component included in the high frequencycurrent 75B at the time of reproduction. Since the high frequencycomponent is not attenuated, the high frequency current generationsection 519 can generate the high frequency current 75B having a smalleramplitude. Then, the radiation of high frequency noise to the outside ofthe semiconductor laser driving apparatus which is caused by the highfrequency current 75B is reduced.

[0171] In addition, the high frequency component included in the highfrequency current 75B to be superposed on a reproduction current 73Bsupplied to the semiconductor laser 21 is increased by the amount whichis prevented from being attenuated. Therefore, the semiconductor lasernoise NL generated in the semiconductor laser 21 is reduced. Thiscontributes to improvement in the signal-to-noise ratio of areproduction signal obtained as a result of high density recording.

[0172] In the above description, the current driving section 511 has afrequency characteristic which enhances the high frequency component.There are cases where the current driving section 511 cannot be providedwith Such a frequency characteristic due to restriction on designthereof. In such cases, the filter 515 and the switching section 520shown in FIG. 3 can be replaced with, for example, the structure shownin FIG. 10,

[0173] A filter 515E shown in FIG. 10 has a frequency characteristic(relationship between the impedance Z and the frequency F) shown in FIG.11A. As shown in FIG. 11A, the filter 515E has a sufficiently lowimpedance Z at the frequency FA (high frequency superposition frequencyof 300 MHz). Owing to the structure shown in FIG. 10, the filter 515Ebecomes active when a switch 516E is switched on and operates asfollows. At the time of reproduction, the filter 515E operates toenhance the high frequency component included in the high frequencycurrent generated by the high frequency current generation section 519(FIG. 3). At the time of recording, the filter 515E operates to enhancethe high frequency component included in the recording current generatedby the recording and reproduction current generation section 518. Theamplitude of the high frequency component of 300 MHz supplied to thesemiconductor laser 21 can be efficiently supplied with the highfrequency component.

[0174] The semiconductor laser driving current noise NK described abovein the section entitled <Problem 2> specifically includes circuit noisegenerated in the semiconductor laser driving apparatus and noisesuperposed on the driving current (recording current or reproductioncurrent) by an external factor from the outside of the semiconductorlaser driving apparatus. Since the semiconductor laser driving currentnoise NK influences a signal band of 20 MHz or lower, the noisecomponent in the signal band of 20 MHz or lower needs to be sent to thefilter efficiently. This requires a filter having a frequencycharacteristic of also passing a signal of 20 MHz or lower as shown inFIG. 13B. However, the filter should not pass a signal of about 100 kHzor lower, since such a signal is used for servo control.

[0175] When a filter for passing a signal of 20 MHz or lower is used,the edges of the pulses included in the recording current are blunted.Therefore, accurate recording may not be realized. As shown in FIG. 12.a switch timing signal S is used to place the filter in an open state atthe time of recording.

[0176] A filter having a frequency characteristic shown in FIG. 11Bsuppresses the noise in a band of 20 MHz or lower during a reproductionoperation, but also undesirably suppresses a high frequencysuperposition signal of 300 MHz, which is necessary for reproduction.Therefore, the filter having a frequency characteristic shown in FIG.11B may not contribute to suppression of the semiconductor laser noiseNL generated in the semiconductor laser 21. In consideration of this, itis desirable to use a filter having a structure shown in FIG. 3 andhaving a frequency characteristic shown in FIG. 11B in combination witha filter having a structure shown in FIG. 10 and having a frequencycharacteristic shown in FIG. 11A. Such a combination provides afrequency characteristic having a sufficiently low impedance (ZA) in asignal brand of 20 MHz or lower and having a sufficiently high impedanceto allow a high frequency current to be superposed on a driving currentof the semiconductor laser at a high frequency superposition frequencyof 300 MHz.

[0177] The specific design the filters 515, 515A, 515B, 515C and 515E interms of frequency characteristics; the positioning of the filters 515,515A, 5155, 515C and 515E and the switching sections 520 and 520A in thesemiconductor laser 21; and setting of the switching timing by theswitching sections 520 and 520A can be determined in accordance withvarious parameters of the system and are not limited to theabove-mentioned examples. When the semiconductor laser driving apparatusincludes a plurality of filters 515A, 515B and 515C and the switchingsection 520A, the circuits in the semiconductor laser driving apparatuscan be appropriately changed so that the plurality of filters can bearbitrarily switched and combined as necessary.

[0178] As described above, the present invention provides asemiconductor laser driving apparatus for providing stable recordingcharacteristics at a high recording density and a high signal-to-noiseratio of a reproduction signal at the time of reproduction, and anoptical disc apparatus including such a semiconductor laser drivingapparatus.

[0179] The present invention provides a semiconductor laser drivingapparatus capable of recording a recording mark having a normal shapeeven when the composition of the optical disc is not uniform, and anoptical disc apparatus including such a semiconductor laser drivingapparatus.

[0180] The present invention provides a semiconductor laser drivingapparatus capable of recording a recording mark having a normal shapeeven when the linear velocity of the optical disc fluctuates at the timeof recording, and an optical disc apparatus including ouch asemiconductor laser driving apparatus.

[0181] The present invention provides a semiconductor laser drivingapparatus capable of recording a stable 3T mark having a normal shape,and an optical disc apparatus including such a semiconductor laserdriving apparatus.

[0182] The present invention provides a semiconductor laser drivingapparatus capable of reducing unnecessary radiation of high frequencynoise to the outside of the semiconductor laser driving apparatus causedby a high frequency current, and an optical disc apparatus includingsuch a semiconductor laser driving apparatus.

[0183] The present invention provides a semiconductor laser drivingapparatus capable of avoiding excessive overshoot or undershoot at therise or fall of the recording current at the time of recording even whena current driving section is provided with a frequency peak in order toenhance the high frequency component included in the high frequencycurrent at the time of reproduction, and an optical disc apparatusincluding such a semiconductor laser driving apparatus.

[0184] The present invention provides a semiconductor laser drivingapparatus capable of preventing the pulses of the recording current frombeing blunted at the time of recording even when a low pass fitter isprovided for reducing the level of noise in the reproduction signal atthe time of reproduction, and an optical disc apparatus including such asemiconductor laser driving apparatus.

[0185] Various other modifications will be apparent to and can bereadily made by those skilled in the art without departing from thescope and spirit of this invention. Accordingly, it is not intended thatthe scope of the claims appended hereto be limited to the description asset forth herein, but rather that the claims be broadly construed.

What is claimed is:
 1. A semiconductor laser driving apparatus fordriving a semiconductor laser for directing light to an optical disc forrecording a recording mark on the optical disc based on a recordingcurrent and reproducing the recording mark recorded on the optical discso as to generate a reproduction signal, the semiconductor laser drivingapparatus comprising: a reproduction current generation section forgenerating the reproduction current; a high frequency current generationsection for generating a high frequency current including a highfrequency component for reducing semiconductor laser noise included inthe reproduction; a recording current generation section for generatingthe recording current, the recording current including a pulsecorresponding to the recording mark and the pulse including a pluralityof multi-pulses: and a current driving section for amplifying thereproduction current and the recording current, wherein the highfrequency component included in the high frequency current generated bythe high frequency current generation section is enhanced at the time ofreproduction, and the high frequency component included in the recordingcurrent generated by the recording current generation section isenhanced at the time of recording, the semiconductor laser drivingapparatus further comprising: a filter for operating so as to attenuatethe enhanced high frequency component included in the high frequencycurrent generated by the high frequency current generation section andthe enhanced high frequency component included in the recording currentgenerated by the recording current generation section; and a switchingsection for switching the filter on or off so that the enhanced highfrequency component included in the recording current is superposed onat least one of the plurality of multi-pulses included in the pulse ofthe recording current.
 2. A semiconductor laser driving apparatusaccording to claim 1, wherein the current driving section has afrequency characteristic for enhancing the high frequency component, andthe current driving section enhances the high frequency componentincluded in the high frequency current generated by the high frequencycurrent generation section at the time of reproduction and enhances thehigh frequency component included in the recording current generated bythe recording current generation section at the time of recording.
 3. Asemiconductor laser driving apparatus according to claim 1, wherein theswitching section includes a switch connected to the filter and a timingcontrol section for controlling the timing of opening or closing of theswitch.
 4. A semiconductor laser driving apparatus according to claim 1,wherein the at least one of the plurality of multi-pulses includes aleading multi-pulse.
 5. A semiconductor laser driving apparatusaccording to claim 1, wherein the pulse includes a specific pulse havinga specific pattern, and the switching section causes the filter tooperate so that the enhanced high frequency component included in therecording current is superposed on the specific pulse.
 6. Asemiconductor laser driving apparatus according to claim 5, wherein: therecording mark includes a 3T mark recorded by 8-16 modulation, thespecific pulse includes a 3T pulse corresponding to the 3T mark, and theswitching section causes the filter to operate so that the enhanced highfrequency component included in the recording current is superposed onthe 3T pulse.
 7. A semiconductor laser driving apparatus according toclaim 1, wherein the switching section causes the filter to operate sothat the enhanced high frequency component included in the recordingcurrent is superposed on a portion of at least one of the plurality ofmulti-pulses included in the pulse.
 8. A semiconductor laser drivingapparatus according to claim 1, wherein the switching section causes thefilter to operate so that the enhanced high frequency component includedin the recording current is superposed on an entirety of at least one ofthe plurality of multi-pulses included in the pulse.
 9. A semiconductorlaser driving apparatus according to claim 1, wherein the at least oneof the plurality of multi-pulses includes a trailing multi-pulse.
 10. Asemiconductor laser driving apparatus according to claim 1, wherein theswitching section causes the filter to operate so that the enhanced highfrequency component included in the recording current is superposed onall of the plurality of multi-pulses included in the pulse.
 11. Asemiconductor laser driving apparatus according to claim 1, wherein thereproduction current is a DC current.
 12. A semiconductor laser drivingapparatus according to claim 1, wherein the switching section causes thefilter to operate so that the enhanced high frequency component includedin the high frequency current is superposed on the reproduction currentat the time of reproduction, and causes the filter to operate so thatthe enhanced high frequency component included in the recording currentis attenuated at the time of recording.
 13. A semiconductor laserdriving apparatus according to claim 1, wherein the high frequencycomponent has a frequency of 100 MHz or higher.
 14. A semiconductorlaser driving apparatus according to claim 13, wherein the highfrequency component has a frequency of 100 MHz or higher and 450 MHz orlower.
 15. A semiconductor laser driving apparatus according to claim 1,wherein the high frequency component has a frequency of substantially300 MHz.
 16. A semiconductor laser driving apparatus according to claim1, wherein the filter includes a high pass filter.
 17. A semiconductorlaser driving apparatus according to claim 1, wherein the high frequencycomponent has a frequency which is higher than a cut-off frequency ofthe filter.
 18. A semiconductor laser driving apparatus according toclaim 1, wherein the filter includes a plurality of filter circuitshaving different frequency characteristics and different impedancevalues from one another.
 19. A semiconductor laser driving apparatusaccording to claim 18, wherein the switching section selects one of theplurality of filter circuits as a filter circuit which operates based ona linear velocity of the optical disc.
 20. An optical disc apparatus,comprising: an optical pickup for recording a recording mark on anoptical disc and reproducing the recording mark recorded on the opticaldisc; a motor for rotating the optical disc and a control block forcontrolling the optical pickup and the motor, wherein: the opticalpickup includes: a semiconductor laser for directing light to theoptical disc for recording the recording mark on the optical disc basedon a recording current and reproducing the recording mark recorded onthe optical disc so as to generate a reproduction signal, and asemiconductor laser driving apparatus for driving the semiconductorlaser, the semiconductor laser driving apparatus including: areproduction current generation section for generating the reproductioncurrent, a high frequency current generation section for generating ahigh frequency current including a high frequency component for reducinga semiconductor laser noise included in the reproduction signal, arecording current generation section for generating the recordingcurrent, the recording current including a pulse corresponding to therecording mark and the pulse including a plurality of multi-pulses, anda current driving section for amplifying the reproduction current andthe recording current, wherein the high frequency component included inthe high frequency current generated by the high frequency currentgeneration section is enhanced at the time of reproduction, and the highfrequency component included in the recording current generated by therecording current generation section is enhanced at the time ofrecording, the semiconductor laser driving apparatus further comprising:a filter for operating so as to attenuate the high frequency componentincluded in the high frequency current generated by the enhanced highfrequency current generation section and the enhanced high frequencycomponent included in the recording current generated by the recordingcurrent generation section, and a switching section for switching thefilter on or off so that the enhanced high frequency component includedin the recording current is superposed on at least one of the pluralityof multi-pulses included in the pulse of the recording current.
 21. Anoptical disc apparatus according to claim 20, wherein the currentdriving section has a frequency characteristic for enhancing the highfrequency component, and the current driving section enhances the highfrequency component included in the high frequency current generated bythe high frequency currant generation section at the time ofreproduction and enhances the high frequency component included in therecording current generated by the recording current generation sectionat the time of recording.
 22. An optical disc apparatus according toclaim 20, wherein: the control block includes a linear velocitydetection section for detecting a linear velocity of the optical disc,and the switching section causes the filter to operate so that theenhanced high frequency component included in the recording current issuperposed on at least one of the plurality of multi-pulses included inthe pulse based on the linear velocity of the optical disc.
 23. Anoptical disc apparatus according to claim 22, wherein the linearvelocity detection section detects the linear velocity of the opticaldisc based on the reproduction signal.
 24. An optical disc apparatusaccording to claim 22, wherein the linear velocity detection sectiondetects the linear velocity of the optical disc based on a rotationspeed of the motor.
 25. An optical disc apparatus according to claim 22,wherein the linear velocity detection section detects the linearvelocity of the optical disc based on a radial position of the opticalpickup on the optical disc.